SS potable water storage tank requirements?

[jlcochran1]

What are the fabrication requirements for stainless steel potable water storage tanks? Is mill surface internal finish acceptable vs polishing? Disinefection required? Any NSF or other standards that would govern tank fabrication?

 

[Montemayor]

jlcochran1:

Unless someone else on this forum has information to the contrary, I have never seen or heard of any surface specifications on Stainless Steel water potable water storage tanks. I've also used Stainless Steel for this application and have followed the same standards as for carbon steel construction. Of course, there should be disinfection! But isn't that a common requirement for ANY material of construction used for potable water applications?

I personally favor SS over FRP or carbon steel because I believe it is:

1) Cost effective; I've found the incremental cost increase over carbon steel is more than worth it when considering the difference in maintenance and fluid cleanliness;
2) It is just as rugged and more durable than carbon steel; it is mechanically sound;
3) Unlike FRP it can withstand mechanical abuse or blows and is even more durable than the FRP;
4) Unlike FRP and CS, it is valid and useful for a multitude of future fluid applications and, as such, is a wiser and more stable investment.

Some people may have contradictory information, but as of yet I have not found any. I hope this experience helps and answers your query.

Art Montemayor
Spring, TX

 

[kenvlach]

I agree with Montemayor.
The ANSI/NSF Standard 61 Certification for Water Contact Material includes such a large variety of materials (e.g., ABS, asphaltic coating, carbon steel, cement, epoxy, galvanized steel... from

http://www.nsf.org/Certified/PwsCom...ype=Coatings+-+Tank&PlantState=&PlantCountry=

and

http://www.nsf.org/Certified/PwsComponents/abbrevs.html)

that the surface finish of the SS can hardly matter. Just clean and sterilize before use.

SS tanks for DI water and process solutions in the pharmaceutical and semiconductor industries are both mechanically polished and electropolished.

 

[quark]

I didn't hear any for potable water tanks but DI and WFI tanks in pharma industry do have a surface roughness value of 230 grit (minimum). The basic reason is that most of the micro living organism fall between 0.4 to 0.2 micron in size. Earlier we used to get mechanically polished but now a days it is only electropolishing (for uniformity of the surface)

It is a well observed fact that biofilms can't be removed by general disinfection procedures. One interesting thing is that organism, living or dead, will create problems almost to same extent. Dead organism become feed to the living ones. That is why(in pharma industry) we take care to avoid settlement of the organism.

We generally epoxy paint the RCC potable water tanks.

Regards,

 

[kenvlach]

Three documents on SS for potable water systems and some additional links:
A British spec. which should be useful: “OPERATIONAL GUIDELINES AND CODE OF PRACTICE FOR STAINLESS STEEL PRODUCTS IN DRINKING WATER SUPPLY”

http://www.euro-inox.org/pdf/map/GuidelinesDrinkingWater_EN.pdf

Pages 24-25 (pdf pp. 30-31) cover surface finish under post-weld cleaning:
“5 Generally, mechanically cleaned surfaces should be taken to at least a 180-grit finish.

6 Pickling would generally be used after mechanical cleaning but can be applied as an alternative to mechanical procedures. Mixtures of nitric and hydrofluoric acid are applied by immersion, by spraying, or locally in paste form. Thorough rinsing is essential and care must be taken not to over-pickle. Safe working practices must be followed. It is important that all pickling residues are removed by thorough rinsing with water. (cf. CP 3.3)

7 For certain, principally hygienic, requirements, a finer mechanical polish followed by electro-polishing is appropriate. Electrocleaning/polishing is an alternative procedure, requiring special power sources and tools.

8 Cleaning and passivation of stainless steels is covered by the specification ASTM A 380 - 99 ‘Cleaning, descaling and passivation of stainless steel parts, equipment and systems’.”

Information on avoiding contamination, cleaning and sterilization similar to earlier posts.

Two documents available from the NICKEL DEVELOPMENT INSTITUTE :
“STAINLESS STEEL PLUMBING” (1997):

http://www.nidi.org/multimedia/tech...re_pdfs/11000_reference_book_series/11019.PDF

Not much detail about finishes, but mentions that peracetic acid and ozone are alternatives to chlorine for sterilization. Warns not to use the sulfur-containing 303 SS.

“Stainless steel for potable water treatment plants,” (1999) is:

http://www.nidi.org/multimedia/tech...erature_pdfs/10000_technical_series/10087.PDF

It generally requires the use of 304L & 316L, or super austenitic or duplex SS at high chlorine levels. Post weld cleaning is similar to above and references ASTM A380, but doesn’t mention final grit size in mechanical polishing.

Lots more SS links, including some on bacteria removal/avoidance as mentioned by quark:

http://www.stainlessarchitecture.org/index.cfm/ci_id/80.htm#north